The present invention relates to a method for recovering a gallium value from an aqueous solution of a crude aluminum salt. More particularly, the invention relates to a method for recovering a gallium value from an aqueous solution of an aluminum salt such as an aqueous solution of crude aluminum salt produced in the Bayer's process with an object to obtain a gallium material and to purify the solution of the aluminum salt relative to the content of gallium as an impurity.
As is well known, gallium is an element increasingly highlighted in recent years as a component element of various electronic functional devices such as light-emitting diodes, semiconductor lasers, field-effect transistors, magnetic sensors and the like and consumption of gallium compounds is rapidly increasing. In addition, the demand for gallium is expected to expand by leaps and bounds in the near future as a component element of so called compound semiconductors to replace traditional silicon semiconductors suitable for use as a high performance solar batteries and substrates of integrated circuits for high speed computers.
Occurrence of gallium is widely found throughout the crust of earth but gallium is a typical dispersed element for which no minerals or ores are known containing the element in such a concentration that the mineral or ore can be used as a natural raw material of the element in a metallurgical or refining process. In other words, any of known gallium resources richest in the content of gallium contains the element of gallium in a much lower concentration than the concentration having economical feasability as a raw material of the element. Accordingly, the resources of gallium currently in use are limited to the materials produced in the processing of a large amount of certain ores containing gallium in a concentration considerably higher than the average in the crust of earth. For example, it is known that a gallium value is contained in a somewhat increased concentration in solutions, precipitates flue dusts and the like in certain processes of chemical treatment of inorganic materials starting from bauxite and several zinc ores as a raw material and these in-process materials can be used as a starting material for obtaining the gallium value. Currently, gallium is obtained in most cases from the so-called Bayer's solutions produced in the manufacturing process of alumina starting from bauxite and precipitates obtained in the metallurgical process of zinc as the principal sources of gallium supply. Unfortunately, the concentration of gallium these sources still very low so that the economical utilizability of these gallium sources largely depends on the efficiency of the technology applied to the enrichment and recovery of the gallium value from the source materials.
The efficiency in the recovery of the gallium value in the prior art method is too low to be economically feasible despite the use of very specific and expensive adsorbents, extractants and solvents and the troublesomeness of the process in addition to the disadvantages of a great loss or consumption of the solvent and extractant and the instability of the adsorbent. Therefore, the technology for the recovery of a gallium value has been established only for the above mentioned gallium-containing materials as a gallium source in which the concentration of gallium is relatively high. For example, Bayer's solutions can be used as a gallium source in the prior art technology only when the content of gallium therein is 100 to 200 mg per liter or higher and no economical method is known for the Bayer's solution of lower gallium concentrations or for the solutions obtained in a low-alkalinity Bayer's process.
As is known, the principal solute in a Bayer's solution is sodium aluminate and, when the solution is neutralized by blowing carbon dioxide gas thereinto to decrease the alkalinity, both of the aluminum and gallium values precipitate in the form of hydroxide or basic carbonates which can be separated from the solution by a known method of solid-liquid separation. The thus obtained liquid portion can be transferred to the process for the recovery of sodium carbonate. The coprecipitates of both of a major amount of the aluminum value and a trace amount of the gallium value are dissolved in sulfuric or hydrochloric acid to give an aqueous solution of aluminum sulfate or chloride as the principal solute and the corresponding gallium salt as a trace impurity.
The above obtained aqueous solution containing the aluminum salt as the principal solute can of course be used as a source material of gallium provided that an efficient method is established for the recovery of the gallium value. Several known methods may be worthwhile for consideration as a method for the separation of the gallium value from an overwhelmingly large amount of the aluminum value including the methods of solvent extraction and anion exchange utilizing the greatly differing behavior of chloro-complex formation between these two elements, adsorption method by the use of a chelate resin, adsorptive resin or inorganic ion exchanger having selective adsorptivity for gallium and so on. These prior art methods are not satisfactory in respect of the efficiency for the recovery of the gallium value if not to mention the economical disadvantage as a consequence of the complicate process and the use of special and expensive solvents, extractants and adsorbents in large quantities. In particular, no practical method worthy to be considered is known for the recovery of the trace gallium value from an aqueous solution containing aluminum sulfate in a high concentration.
As a general guide principle, the method of precipitation utilizing a precipitation reaction is industrially the most Preferable for the recovery of a trace ingredient from an aqueous solution because the process is simple and convenient and suitable for the processing of a large volume of the solution if the precipitation reaction is complete by using a relatively small amount of the precipitant and no particular difficulties are encountered in the solid-liquid separation of the precipitates from the mother liquor. When recovery of a trace amount of a gallium value is intended from an aqueous solution of an aluminum salt in a high concentration, however, no practical precipitation method is known because the selective precipitation of the gallium value can never be complete as a consequence of the great similarity in the precipitation behavior of these two elements belonging to the same group in the Periodic Table.
It has long been known in the analytical chemistry since nearly a century ago that gallium ions can be precipitated from an aqueous solution by the addition of a soluble ferrocyanide. It is generally understood that, when selective precipitation of gallium is desired from an aqueous solution in the coexistence of aluminum ions, the precipitation reaction must be conducted in the presence of hydrochloric acid in a high concentration because aluminum ferrocyanide is not precipitated in an aqueous solution containing hydrochloric acid in a high concentration as is reported by P. B. Browning and L. E. Porter in American Journal of Science, volume 44, pages 221-224 (1917). In other words, this knowledge in the analytical chemistry teaches that coprecipitation of aluminum ferrocyanide with gallium ferrocyanide would predominantly take place when the concentration of hydrochloric acid is low in the aqueous solution. At least no attempts have been successful in recovering a gallium value by the precipitation method from an aqueous solution containing a large amount of an aluminum salt.